(1) Field of the Invention
The present invention relates to a composite sintered compact comprising a sintered body, which contains at least one of high density boron nitride and diamond, and a cermet or a metal bonded to the sintered body, which sintered compact is used as a cutting tool, draw die, wear-resistant material and the like. The compact superior to conventional composite sintered compacts in the bonding strength and is easily sintered during its production.
(2) Related Art Statement
Composite sintered compacts are produced due to the following two reasons.
(1) A sintered body portion cannot be brazed to other material, and hence a substrate is bonded to the sintered body portion, so that the resulting composite sintered compact can be brazed to other material at the substrate portion. PA1 (2) A sintered body portion consists of a high-hardness material, and hence the sintered body portion is strong, but is brittle. Therefore, the sintered body portion is lined with a cermet or a metal, both of which have a toughness higher than the that of the sintered body portion, to produce a composite sintered compact having a high toughness as a whole. PA1 (1) A structure obtained by a method, wherein a sintered body portion containing a metal is simply superposed on a substrate portion, and the superposed mass is sintered to dissolve mutually the metal contained in the sintered body portion and the metal contained in the cermet of the substrate portion or the metal contained in the substrate portion. PA1 (2) A structure obtained by a method, wherein a sintered body portion is superposed on a substrate portion through a metal or cermet interposed between them, said metal or cermet being used independently of the kinds of the sintered body portion and substrate portion, and the superposed mass is sintered to bond the sintered body portion to the substrate portion through the metal or cermet. PA1 (3) A structure obtained by a method, wherein a sintered body portion containing no metal is superposed on a cermet, and the superposed mass is sintered such that a metal contained in the cermet is diffused by the capillary action into the space between the powders of the high density boron nitride and/or diamond, whereby he fellow powders of the high density boron nitride and/or diamond are bonded to each other, and at the same time the cermet portion is bonded to the sintered body portion. PA1 (1) The intermediate layer has an affinity to both the sintered body and the substrate. PA1 (2) The intermediate layer has a thermal expansion coefficient approximate to those of the sintered body and the substrate. PA1 (3) The intermediate layer has a high strength over a wide temperature range. PA1 (4) The intermediate layer has a high toughness and has high resistances against impact stress and repeating stresses.
The bonding structure of the sintered body portion with the cermet or metal portion (hereinafter, referred to as substrate) in the conventional composite sintered compact can be classified as one of the following three types.
Any of composite sintered compacts produced at present have fundamentally the same structure as one of the above described three kinds of structures, a structure which is a little modified from the above described structures, or a structure of a combination thereof.
In the production of a composite sintered compact, a proper structure is selected from the above described three kinds of fundamental structures depending upon the use purpose and use condition of the resulting composite sintered compact, the manufacturing technic and manufacturing apparatus possessed by the manufacturer, and other conditions, and it can not be affirmed which of the above described structures is superior or inferior. However, there is a common trouble to all structures due to the structure formed by the bonding of different kinds of materials. That is, a composite sintered compact is exposed to a temperature ranging from room temperature to one thousand and several hundreds degree in centigrade during the processes from its production to its consumption, that is, the process of the production of the composite sintered compact through sintering, process of the working of the composite sintered compact into manufactured goods, such as cutting tool, draw die and the like, by its brazing to the based metal for cemented carbide or the like, and the process of the use of the product. In this case, the sintered body and the substrate are different in thermal expansion coefficient from each other due to the difference in the material constituting them, and hence the composite sintered compact is exposed to a stress caused by the difference in the thermal strain at every temperature between the sintered body and the substrate. When the substrate is made of cermet, it is possible to produce a substrate having a thermal expansion coefficient near that of the sintered body portion by adjusting the amount of the ceramic portion and that of the metal portion. However, such an adjustment is possible only at a specifically limited temperature, and it is impossible to carry out the adjustment over a whole temperature range, to which the composite sintered compact is exposed, due to the reason that the sintered body is different from the substrate in the change of thermal expansion coefficient due to the temperature change. When a substrate made of metal is used, the thermal expansion coefficient of the substrate can be adjusted to that of the sintered body portion only at a specifically limited temperature as well as the case of the substrate made of cermet. However, it is impossible to effect the adjustment over a whole temperature range, to which the composite sintered compact is exposed. Moreover, even when the requirements relating to the thermal expansion coefficient are attained, other requirements, such as heat resistance, strength and affinity of the substrate to the sintered body portion in the bonding, can not be substantially satisfied.
As described above, there are various problems in the conventional composite sintered compacts. First, there is always a risk of formation of cracks, according to the mechanism explained hereinafter, in all the processes from the production of the composite sintered compact to the consumption thereof due to the thermal stress caused therein by the difference in the thermal expansion coefficient between the sintered body portion and the substrate due to the bonding of different materials.
(1) A composite sintered compact is produced by a sintering under a certain constant temperature and pressure condition, and after completion of the sintering, the temperature and the pressure are reduced to room temperatures and normal pressures. In this case, the composite sintered compact, which has been sintered while maintaining the equilibrium in the shape under the certain constant temperature and pressure condition, is exposed to a severe temperature and pressure change of from one thousand and several hundreds degree in centigrade to room temperature and from several tens thousands atmospheres to normal pressures in a very short period of time of from several minutes to several tens of minutes. Therefore, the difference in the thermal expansion coefficient between the sintered body and the substrate is varied corresponding to the temperature change during the cooling, and hence the thermal stress caused in the sintered body is different from that caused in the substrate. Moreover, the sintered body is different from the substrate in modulus of elasticity, and hence the sintered body is different from the substrate in the deformed amount of shape corresponding to the change of pressure. As a result, a very complicated strain is caused in the resulting composite sintered compact. Accordingly, the prevention of the development of cracks in the composite sintered compact due to the stress caused therein has been a problem which requires a complex technique in the production of the composite sintered compact.
(2) Even when the above described problem has been solved and a good composite sintered compact free from cracks has been produced, the composite sintered compact is brazed at its substrate to other materials to obtain a manufactured good in many cases. The brazing temperature depends upon the melting temperature of the solder, and the brazing is generally carried out at a temperature of not higher than 800.degree. C. In this case, the brazing to other material is carried out under normal pressures, and therefore although stress due to the variation of pressure is not caused, thermal stress is naturally caused, and further stress is caused in th composite sintered compact due to the difference in thermal expansion coefficient between the composite sintered compact and the material, to which the composite sintered compact is brazed. For example, when a composite sintered compact is brazed to a steel having a carbon content of 0.06%, the steel has an average linear thermal expansion coefficient of about 13.times.10.sup.-6 /.degree. C. within the temperature range of 0.degree.-300.degree. C., but a tungsten carbide-cobalt alloy, which is one of cermets to be predominantly used for substrates, has a low linear thermal expansion coefficient of (4-7).times.10.sup.-6 /.degree. C. in a wide cobalt content range of 3-20% by weight. Moreover, the thermal expansion coefficient of the sintered body portion varies depending upon the kind and amount of materials to be added to the basic component of the sintered body portion at the sintering. When high density boron nitride and/or diamond are used as the basic component, the thermal expansion coefficient of high density boron nitride is a very low value of 4.8.times.10.sup.-6 /.degree. C. at 430.degree. C., and those of diamond are also very low values of 1.5.times.10.sup.-6 /.degree. C. at 78.degree. C. and 3.5.times.10.sup.-6/ .degree. C. at 400.degree. C. Accordingly, the difference in the thermal expansion coefficient between the sintered body and the substrate are large in the whole constitution of a cutting tool using the composite sintered compact, and a complicated thermal stress is caused in the composite sintered compact. Therefore, there is a high risk of formation of cracks in the composite sintered compact.
(3) Cutting is an operation which imposes the severest condition on a composite sintered compact during its use among other operations carried out by use of the composite sintered compact. A composite sintered compact containing diamond is predominantly used in the cutting of non-ferrous metal under a low load, and is not used many times in the cutting under severe conditions. However, almost all composite sintered compacts containing high density boron nitride are used in the cutting of iron, steel or ferrous alloy and are used very often in interrupt cutting under a high load. In such a case, the cutting edge-temperature generally reaches a high temperature of about 800.degree. C., and hence the composite sintered compact is exposed to a very severe condition caused by a synergistic effect of thermal stress due to the high temperature, impact load and vibration, and cracks often are formed.
The type of stresses caused in a composite sintered compact mainly due to heat during the above described three kinds of processes from its production to its consumption are not uniform, and it is difficult to solve by the same simple method the troubles caused in these three processes. However, a common problem caused in these three processes is a thermal stress caused by the bonding of different materials into a monolith composite sintered compact.
In order to solve this problem, it has been thought of producing a composite sintered compact consisting of a sintered body and a substrate, both having the same thermal expansion coefficient over the whole temperature range. However, it is impossible to produce such composite sintered compact as described above. Further, it has been thought that the above described problem might be solved by producing a sintered body and a substrate, both having a strength sufficiently higher than the thermal stress to be generated therein. However, although it may be possible to produce a sintered body and a substrate, both having a strength sufficiently higher than the thermal stress generated therein within a room temperature range, it is substantially impossible to produce a sintered body and a substrate, both having a strength sufficiently higher than the thermal stress generated therein within a high temperature range which lowers the strengths of the sintered body and substrate. Therefore, it is impossible to prevent the formation of cracks in a composite sintered compact within the whole processes from its production to its consumption.
As a final means, there can be thought of a method, wherein a material different from both of a sintered body and a substrate is interposed between them in order to act to bond the sintered body with the substrate and to relax thermal stress and other various stresses. This method corresponds to method (2) among the above explained three kinds of methods for bonding a sintered body with a substrate. However, whether the different material (hereinafter, referred to as an intermediate layer), which is interposed between the sintered body and the substrate, is a cermet or a metal, the following problems still remain. The intermediate layer is required to have the following properties.
It is very difficult to produce an intermediate layer which can satisfy all of the above described requirements. Particularly, it is very difficult to produce an intermediate layer which can satisfy the requirement (4). In the conventional composite sintered compact, the sintered body has hitherto been very often peeled off from the substrate at the intermediate layer or at the vicinity thereof under a severe cutting condition.
In order to solve the above described problems, the inventors have made various theoretical and experimental investigations for a long period of time with respect to the means and structures for bonding the sintered body to the substrate, and reached the present invention.
The theoretical ground of the present invention will be explained hereinafter.
As described above, the sintered body and the substrate are essentially different in object from each other. Therefore, it is desirable to produce a sintered body and a substrate from different materials, each having a property which satisfy the object of each of the sintered body and the substrate, respectively. Moreover, it is necessary that both the materials can be bonded with each other, and the bond between both the materials can resist the above described very severe condition. It is very difficult to bond two materials, which have different properties and objects from each other, by their own bonding ability under a very severe condition without causing stress, and hence composite sintered compacts having no drawbacks have not hitherto been obtained. When a sintered body is bonded to a substrate through an intermediate layer, which acts to bond the sintered body with the substrate, it is not necessary to use a sintered body and a substrate, both of which have mutually a bonding ability in themselves, and therefore it seems that a more excellent composite sintered compact can be obtained. However, there has not yet been obtained an excellent material, which is adapted to be used in the intermediate layer and can satisfy the above described requirements. Therefore, it has been difficult to produce a composite sintered compact having excellent properties by bonding a sintered body with a substrate through an intermediate layer interposed, therebetween. However, when an excellent material to be used as an intermediate layer can be discovered, a composite sintered compact having the most excellent bonding structure can be produced as described above by interposing an intermediate layer between the sintered body and the substrate. Therefore, in the present invention, the composite sintered compact is limited to a composite sintered compact having a structure, wherein an intermediate layer is interposed between a sintered body and a substrate, and investigations have been carried out with respect to the affinity and bonding strength of various materials which are to be used in the intermediate layer, to two materials which are to be used in the sintered body and in the substrate, and further with respect to the toughness against various stresses and the heat resistance of various materials which are to be used as the intermediate layer, during the processes ranging from the production of the composite sintered compact to the consumption thereof.
First, an explanation will be made with respect to the affinity of materials to be used as the intermediate layer. A sintered body contains high density boron nitride and/or diamond as a main component. That is, boron which is an element constituting high density boron nitride has an affinity to high density boron nitride, and hence it is easy that a material, which contains boron as one of the elements constituting the material, wets high density boron nitride, or forms a compound or a solid solution together with a high density boron nitride. As materials other than the material containing boron, there are advantageously used materials containing Ti, Zr, Hf, Al, Mg or Si, because these elements have a wettability with high density boron nitride. When a sintered body consists of diamond, materials containing Fe, Co, Ni, Cr, Mn, Mo, Ta, Nb, Cu, Au or Ag are advantageously used as the intermediate layer. Because, these elements can form a solid solution with carbon at high temperatures and pressures and have a high wettability with carbon as is understood from the phenomena that these elements are used alone or in combination with other metals as a melting solvent in the synthesis of diamond or as both of a melting solvent and a filler in the sintering of diamond.
Then, an explanation with respect to the bonding strength, toughness and heat resistance of the material to be used as the intermediate layer will be explained. These properties are automatically determined depending upon the kinds of the materials to be used as the intermediate layer, and it is not proper that these properties are separately discussed. Generally speaking, there have hitherto been used metals, such as Ti, Zr, Cu, Mo, W, Ni, Co and the like, and cermets containing carbide or nitride of these metals. The drawbacks of these metals and cermets will be explained, and further an explanation will be made with respect to the intermediate layer having a desired property in order to overcome the drawbacks of these metals and cermets. Ti and Zr are substantially satisfactory with respect to the bonding strength and heat resistance. However, in the production of a composite sintered compact, a sintered body and a substrate are bonded through Ti or Zr at high temperatures under high pressures, and therefore Ti or Zr is apt to react with high density boron nitride and/or diamond in the sintered body to form boride, nitride and carbide of Ti or Zr. When the amount of these metals is large, although a composite sintered compact having high strength and heat resistance is obtained, the composite sintered compact is brittle. Moreover, the amount of Ti or Zr is difficult to control, and therefore the sintered body is often peeled off from the substrate after sintering in the production of a composite sintered compact or during the use of the resulting composite sintered compact. Moreover, both Ti and Zr are high-melting point metals, and therefore the bonding of a sintered body with a substrate through Ti or Zr is difficult unless the sintering temperature is high. Cu has a low melting point, and therefore Cu has a good workability. Moreover, Cu has a high affinity to both of various kinds of sintered bodies and substrates. However, Cu has such a drawback that Cu is softened, due to its low melting temperature, by the high temperature during the use of the resulting composite sintered compact, and hence the sintered body is easily separated from the substrate. Mo and W have a very high melting point contrary to Cu, and therefore it is difficult to bond a sintered body with a substrate through the Mo or W intermediate layer, and moreover even when a sintered body is bonded with a substrate through the Mo or W intermediate layer, the Wo or W intermediate layer is poor in toughness. When cermet is used as an intermediate layer, the cermet intermediate layer is poor in toughness similarly to the case of W and Mo.
In order to solve the above described problems, the inventors have made various theoretical and experimental inventions with respect to the properties, which are demanded of the intermediate layer, and to the materials suitable to be used as the intermediate layer, and have found out the following facts. That is, when an amorphous metal is used as an intermediate layer, particularly when an amorphous metal containing at least one element selected from the group consisting of Ti, Zr, Hf, Fe, Co, Ni, Cr, Mn, Mo, Ta, Nb, Cu, Au, Ag, B, Al and Si, is used as an intermediate layer, the intermediate layer can exhibit excellent performance for all the above described various demands.